The battery could not care any less what you plug in once the engine is running. Its job is basically done after cranking. The alternator is what provides all the current you use, and because of all the sensitive computers, it's WAY over-rated for typically needs.
Like Docfixit said, it's the sockets you don't want to overload. Fuses will pop before you can draw enough current to do damage, but here's a point of interest. The power supplies for a laptop computer or a dvd player do not draw as much current as is listed on their labels. That listed current is the "surge" current when first plugged in. After that, the constant current is much lower, often less than half. If your power outlets stay live all the time, just plug in one item at a time until they're all plugged in and running. If your power outlets go dead when the ignition switch is turned off, all the items will turn on a have a big surge at the same time when you turn on the ignition switch. That could repeatedly pop fuses. In that case, you will have to unplug some items, then plug them in one at a time.
Also, when using power inverters, volts times amps going in roughly equals volts times amps going out. Since output voltage is 10 times higher going out, output current will be one tenth as much as going in. That means that for every amp your inverter supplies to run items, ten amps are going through the power outlet and fuse. If your laptops can run on 12 volts directly, (most can not), they will draw less current. Their power adapters don't follow the same conversion efficiency as inverters so they add to the demand from the inverter. I suspect the navigation unit will draw very little current and it will probably run on the car's 12 volts system directly. You might consider plugging that one into the cigarette lighter if you have one. That outlet has a mechanical thermal release that will not be a factor, and it has a thermal cutout built into the socket as a safety measure. The power outlets only have thermal cutouts, but they respond much slower than fuses, and only in the case of long term overheating, not short surges.
Monday, May 4th, 2009 AT 3:55 AM